In a preliminary study for internal perfusion experiments, a series of experiments was carried out in which efflux was measured from barnacle giant muscle fibers injected with 14C-labeled 3-O-methylglucose (3-O-MG). Phloretin was found to be a much more potent inhibitor than phlorizin, thus indicating that the sugar transport system is purely passive and does not involve the cotransport of Na ion. 3-O-MG efflux is greatly decreased by a high internal concentration of D-glucose, but was unaffected by a high concentration of L-glucose. Subsequent injection of EDTA, EGTA, or high K ion solutions did not effect 3-O-MG efflux. Injections of apyrase of phosphodiesterase markedly lowered 3-O-MG efflux, thus indicating possible roles for ATP and/or cyclic nucleotides in the control of sugar permeability. By using high concentrations of inhibitors, the leakage of 3-O-MG was found to average 22% of the basal rate of efflux.